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Enhancing Diversity Gain in OFDMA for Robust Transmission

This submission discusses the optimization of OFDMA numerology to enhance diversity gain for non-contiguous resource allocation. The study emphasizes the importance of robust transmission in OFDMA DL MU UL, focusing on multicast trigger frames and non-contiguous resource allocation strategies. Simulation parameters and examples are provided, showing how diversity gain can be maximized by spreading non-contiguous resource allocation over frequencies and configuring limitations on allocation. Various sub-block configurations are evaluated, with insights on performance gains and limitations. Signaling aspects and candidate scenarios for non-continuous RU allocations are explored to improve system efficiency.

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Enhancing Diversity Gain in OFDMA for Robust Transmission

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  1. Diversity Mode in OFDMA • Date:2015-11-09 Authors:

  2. Background • OFDMA numerology is optimized considering tradeoff between OFDMA gain and signaling overhead for 11ax • In this submission, diversity gain is shown to discuss about feasibility for non-contiguous resource allocation

  3. Need for Robust Transmission in OFMDA DL MU UL MU preamble preamble STA 1 (BA+Data) STA 1 (Data+unicast TR) • Multicast Trigger Frame is used in cascaded MU operation. • Given the small payload size of TR (compared with Data frames) it is likely to be allocated with small RU (e.g. 26 or 52 RU). • The nature of multi-cast creates difficulty in obtaining frequency selective gain since it will be difficult to choose a channel that has good link conditions for multiple STAs simultaneously. • The narrow RU size and random selection of frequency position makes the Trigger frame susceptible to packet reception loss from channel deep fading. Currently, narrow RUs have no diversity gain. … STA 2 (BA) Multicast TR … STA 2 (Data+unicast TR) STA 4 (Data) STA 5 (Data) STA 3 (Data+unicast TR) STA 3 (BA + Data)

  4. Non-Contiguous Resource Allocation • Diversity gain can be achieved by non-contiguous resource allocation spread over frequency • Design complexity can be reduced by configuring some limitation on non-contiguous allocation • Keep the number of data/pilots tones identical to contiguous RUs • No introduction of new BCC/LDPC interleaver sizes For example) N/2-tones IFFT N-tones Channel coding MOD N/2-tones

  5. Simulation Parameters • Bandwidth : 80MHz • Multi-antenna transmission : 1x1 and 4x1 • MCS: 0, 3 (code rate ½) and 4 (code rate ¾ ) • Payload Size: 100 bytes • Channel: TGacChD • Carrier frequency offset (CFO): fixed at 40 ppm (@ 5GHz) • Phase noise (both at Tx/Rx): -41dBc • Real timing estimation & synchronization • Diversity tested using evenly spaced subblocks • Examples shown in next slide

  6. Examples of Sub-blocks Used for Simulation • Subblocks of a Non-contiguous RU is chosen to be equally spaced apart within 10/20/40/80 MHz. • Number of pilots for a given RU is split equally between subblocks. • 1 pilot per subblock, 2 subblocks for 26 RU • 2 pilot per subbblock, 2 subblock for 52 RU • etc N N N/2 N/2 N/4 N/4 N/4 N/4 10/20/40/80 MHz 10/20/40/80 MHz

  7. Two Sub-block 26/52 RU- 1x1 Tx-Rx, MCS 0/3/4 0.7 dB 2dB 1 dB 1.7 dB 2-3 dB 2 dB MCS 4 MCS 4 MCS 3 MCS 0 MCS 3 MCS 0 • Diversity gain is expected to be the largest when code rate is ½ or when DCM is used. • Around 2 dB gain observed even with 10MHz separation • Performance saturated after 10MHz separation

  8. Two Sub-block 106/242 RU- 1x1 Tx-Rx, MCS 0/3/4 MCS 4 MCS 4 MCS 3 MCS 3 MCS 0 MCS 0 • Two sub-blocks for 106 RU & 242 RU • Diversity gain are smaller for 106 RU and 242 RU (compared with 26 and 52 RU).

  9. Four Sub-block 106/242 RU- 1x1 Tx-Rx, MCS 0/3/4 1.5 dB 0.7 dB MCS 4 MCS 4 MCS 0 MCS 0 MCS 3 MCS 3 • Four sub-blocks for 106 RU & 242 RU • 1.5 dB performance gain for 106 RU. • Diversity gain seems to be minimal for 242 RU.

  10. Simulation Summary 2 2 2 2 26 26 1 1 1 1 1 1 1 1 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 *Based on 1 Tx, with MCS between 0 ~ 4 1 1 1 1 2 2 2 2 1 1 1 1 52 52 52 52 52 52 52 52 26 26 • With small RU (e.g. 26 RU and 52 RU), up to 3.5 dB gain can be achieved • As expected small RUs have the most diversity gain. Large RUs already span wide enough to capture most of diversity gains. • Given hierarchical structure of OFDMA numerology, 4 sub-blocks may give scheduling limitation (e.g. large fragmentation of the resources) • Limiting the diversity mode to 26 and/or 52 RU may be sufficient. 1 1 1 1 26 26 102+4 102+4 102+4 102+4 1 1 1 1 242 242 Example on 40MHz Once 4 green colored sub-blocks of 26 tones are assigned for a STA, rest red colored 52/106/242 tones cannot be assigned to other STAs in OFDMA > Results in fragmentation of the resources

  11. Signaling Aspects of Non-Continuous RU • Given that performance benefits of non-continuous RU is focused on 26/52 RU with at least 10MHz or more separation, we can define non-continuous RU allocations with the reserved states. • For example, only support non-continuous allocation for 26 and/or 52 RU • Excellent candidates for non-continuous RU are the central 26 RUs in 20/40/80 MHz. • 8 bit RU allocation field in SIG-B only may to signal ~180 some states. This means there are additional ~70 states reserved.

  12. Example Candidates for Non-Continuous RU Case 3) 52 52 26 Case 1) Case 2) HE80 • Two separate 26 RUs can be combined into a single non-continuous 52 RU. • Two 13 tones of 26 RUs (i.e. half) can be combined into a single non-continuous 26 RU. • The other half can form another non-continuous 26 RU.

  13. Straw Poll 1 Do you agree to add the following to the SFD? • Transmission diversity mode (i.e. non-continuous transmission) shall be supported in 11ax. • Transmission diversity mode divides a single encoded packet in half and maps to 13 + 13 (26 RU) or 26 + 26 (52 RU) tones, that are spaced apart in frequency. • TBD whether only 26 RU, only 52 RU, or both 26 and 52 RU support transmit diversity mode. • Y/N/A

  14. Yujin Noh, Newracom References • [1] 11-15/0132r9, Specification Framework for TGax • [2] 11-15/1066r0, HE-SIG-B Contents

  15. Appendix

  16. Two Non-Contiguous 13 tones for RU26(examples for simulation) 10M 20M 40M 80M

  17. Two Non-Contiguous 26 tones for RU52(examples for simulation) 10M 20M 40M 80M

  18. Four Non-Contiguous 13 tones for RU52(examples for simulation) 20M 40M 80M

  19. Two Non-Contiguous 53 tones for RU106(examples for simulation) 20M 40M 80M

  20. Four Non-Contiguous 26/27 tones for RU106(examples for simulation) 20M 40M 80M

  21. Two Non-Contiguous 121 tones for RU242 Four Non-Contiguous 60/61 tones for RU242(examples for simulation) 40M 80M 40M 80M

  22. Non-Contiguous RU52(Tx 1 and MCS 0) 3-3.5 dB

  23. Non-Contiguous RU26 (TX1/Tx4 and MCS 3) TX 4 TX 1 • 2 sub-blocks for 26 RU • Diversity gain seems to be decreased with increased number of Tx antenna

  24. Non-Contiguous RU52 and RU106(Tx1/Tx 4 and MCS 3) TX 4 TX 4 TX 1 TX 1

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